1. Field of the Invention
The present invention relates generally to a method and apparatus for applying a thin film coating to an article and more particularly to a method and apparatus for providing a thin film coating to a portion of an article while substantially isolating other portions of the article from the coating environment. In a preferred embodiment, the present invention relates to a method and apparatus for applying a thin film coating to the front face of a cathode ray tube (CRT) after assembly using a thin film deposition technique such as sputtering.
2. Summary of the Prior Art
Although the invention has general application to the thin film coating of an article through a variety of thin film deposition techniques such as electron beam deposition, chemical vapor deposition and sputtering, among others, it has particular applicability to the application of a thin film anti-reflective or other coating onto the front face or screen of a CRT after assembly.
A major objective of designers and manufacturers of displays using CRTs is to reduce glare resulting from the reflection of ambient light off the CRT face. Several approaches have been used in the prior art to achieve glare reduction on CRT screens. One approach has involved surface treatment of the screen by chemical etching such as by means of a hydrofluoric acid solution. Examples are disclosed in U.S. Pat. Nos. 3,679,451 issued to Marks et al. and 3,941,511 issued to Deal et al. Both methods seek to reduce glare from the CRT face by providing a treated surface which scatters incident light while still maintaining good transmittance of light emanating from the CRT face. In general, however, anti-reflective coatings applied through chemical etching achieve only minimal glare reduction and usually result in degradation of the resolution.
A further approach to glare reduction has been to provide a CRT with an anti-glare filter consisting of a piece of glass or other material having an anti-reflective view surface. The filter is placed in a frame and suspended in front of the CRT view surface. In such a device, the glass filter may be tinted or bear an absorbing coating to provide contrast enhancement. Such a device is known as a contrast enhancement filter. Coatings onto the glass filter may also be in the form of optical interference coatings applied to the glass surface by means of physical vapor deposition methods such as sputter and evaporative deposition. They may also be applied by means of chemical vapor or by liquid deposition methods such as spin or dip coating.
A third approach has been to apply optical interference coatings to a CRT screen prior to assembly into a finished unit. For such method to be successful, however, the applied coating must be able to survive the subsequent processing steps during assembly of the unit. The most challenging of these subsequent processing steps is the "frit sealing" step in which the face plate is sealed to the funnel of the CRT by using a paste comprised of glass and ceramic particles. The temperatures needed for the frit seal process may be as high as 450.degree. C. Many optical interference coatings will undergo an irreversible and deleterious alteration of their properties on exposure to these processing conditions. Such changes may also alter optical thicknesses and electrical conductivity optical constants of several of the layers, thereby resulting in a loss of desired optical or electrical conductivity properties.
A still further approach known in the art for providing a CRT screen with anti-reflective properties is to coat a piece of glass with an anti-reflective coating and then bond the glass directly to the CRT. Such a process is known in the art as bonded panel construction. Such processes are expensive since they require a precision bent glass substrate and can result in significant yield loss because of the CRTs and panels which must be discarded due to imperfections in the process.
A desirable feature of anti-reflective coatings intended for CRT face plates or glare filters regardless of the application process, is electrical conductivity. Such conductivity should preferably be sufficient to facilitate the dissipation of static electrical charges and thereby reduce accumulation of dust on the CRT or filter. Electrically conductive coatings are not possible with methods involving chemical etching. Even with the other processes described above, where electrically conductive coatings are possible, additional time consuming processing steps must be undertaken to electrically connect the coating to the implosion band or other grounding component so that the static charges can be dissipated.
Attempts to directly coat the face plates of CRTs or other similar articles after assembly have not proven to be successful. Several reasons exist for this. First, many of the materials and components in the finished CRT are not compatible with the conditions existing in a thin film deposition environment such as, for example, magnetron sputter deposition environments. Second, an assembled CRT embodies various polymeric materials including electronic and other components at the rear of the CRT. These tend to "outgas" or release volatile contaminants when subjected to the heat, vacuum and ion bombardment of thin film deposition environments. Such volatile contaminants may include water vapor, plasticizers, solvents and oligomers. The presence of these outgas components adversely affect the coating process and operation of the deposition equipment. This in turn adversely affects the quality and characteristics of the anti-reflective coating. Although outgassing can be reduced by exposing the assembled CRT to vacuum conditions for an extended period prior to coating, this is time consuming and expensive.
Accordingly, there is a need in the art for an improved method and apparatus for providing the face plate or screen of CRTs and other similar articles with an anti-reflective or other coating which is cost effective and which overcomes the problems currently existing in the prior art. A more specific need exists for a method and apparatus for directly coating a CRT face plate or other article after assembly with a highly acceptable coating without regard to interference by outgassing from assembled CRT components and without regard to the incompatibility of such components to the deposition environment.